This invention relates to the method of producing a pneumatic tire having an apex filler, the method and apparatus for producing a bead and apex filler subassembly, and the tire produced using the bead/apex filler subassembly for use in a pneumatic tire. More particularly, the apex filler is produced from an extruded strip of elastomeric material, which is then reformed after extrusion so as to reduce the stress build-up in the apex filler caused by wrapping the filler around an annular bead.
A pneumatic tire is composed of a tread, a belt structure, and a carcass comprising one or more plies of fabric cords coated with an elastomer. The tire contains axially spaced beads which define the innermost diameter of the tire, each bead comprising a metallic, annular ring, coated with an elastomer, adapted to contact and engage one rim of a wheel. Each bead may be combined with other elements such as flippers, chippers, apex fillers, toe guards and chaffers, all of which serve specific functions in the construction and/or the performance of the tire. This combination of the annular bead and the apex filler serves the purpose of providing a smooth transition between the bead and the adjacent sidewall of the tire. Often, the bead and apex filler are formed as a subassembly around which the carcass ply or plies are wrapped.
One prior art method of fabricating the combined bead and apex filler subassembly involves applying a flat strip of an elastomer along the inner circumference of the bead, and then encapsulating the bead by wrapping the elastomer strip around the bead. The portion of the elastomer strip which extends radially outwardly from the bead serves as the apex filler. This procedure has inherent difficulties, particularly in splicing the abutting ends of the wrapped strip. When the elastomer strip is wrapped around the bead, the outer circumference of the elastomer strip must be stretched to accommodate the difference in the circumferential dimension of the inner periphery of the strip, which is disposed about and abutted against the bead, and the outer periphery of the strip. This stretching induces stresses which can produce warping of the radial outer portion of the apex filler due to the elastic memory of the filler. The degree of warping from one apex filler to the next is not easily controllable, and this factor adversely impacts the consistency of the tire building operation and the uniformity of the resultant tires.
Another method for making the bead/filler sub-assembly is to apply the apex filler to a rotating bead for approximately one revolution after which the apex filler is cut and the two ends are gripped and stretched to splice them together. In like manner as before, this stretching causes the radially outermost edge of the filler to be elongated more than the inner portion, thus leading to warpage and/or cupping of the filler with the outer edge curling radially inwardly and axially away from the bead.
Yet another method is described in U.S. Pat. No. 5,203,938. This patent describes the use of a chuck assembly with a pair of powered, adjustable rollers which engage the two sides of an elastomeric filler as it exits an extruder. The rollers serve to apply a variable speed to the filler, with a first, slower speed being applied to the base of the filler, and a second, faster speed applied to the top as the strip is rotated into an annular shape corresponding to the outer periphery of the bead. The differential speed serves to minimize distortion of the filler. The operation can be used to apply the apex filler directly to a bead positioned in a chuck.
Another procedure is to lay up the tire carcass plies on a tire building mandrel, set the annular beads at the shoulders on either side of the mandrel, place an extruded apex filler around the mandrel at each side thereof, and wrap the ends of the green carcass plies around the beads and the fillers. The sidewall, tread and breakers are wrapped around the green carcass after which the green carcass is placed in a mold and is formed under heat and pressure into a toroidal shape and is cured. In the shaping and curing process, the apex fillers are rotated 90xc2x0 from a flat position on the mandrel to a vertical, radial position. This method of tire construction can cause air to become trapped between the apex filler, the bead, the carcass plies and the turn-up, thereby leading to the formation of blisters in the tire sidewall and the premature failure of the tire. Furthermore, the turning of the apex filler around the curved sidewall of the tire causes the base of the filler to push compressively against the bead and the top of the apex filler to stretch to a larger diameter, thereby causing it to be in tension which leads to possible distortion and irregularities in the tire construction previously described.
It is an object of the present invention to provide a method and apparatus to pre-assemble a bead and apex filler for a tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
It is a further object of the present invention to permit the pre-assembly of a bead and an apex filler, with the apex filler in a stress neutralized state.
Another object of the present invention is to construct a subassembly of a bead and an apex filler wherein the apex filler is initially prestressed on a mandrel as it leaves the extruder and after which the stresses within the apex filler are at least partially neutralized as the filler is placed around the outer periphery of the bead.
Another object of the present invention is to increase the uniformity in the bead/apex filler subassembly step of the tire building process and in the pneumatic tires themselves.
These and other objects and advantages, which will become readily apparent from the following disclosure, are achieved in the construction of a pneumatic tire as hereinafter described.
In accordance with the present invention, a method is described for producing a pneumatic tire which comprises a tire carcass with a pair of annular beads adapted to engage the rims of a wheel. The carcass comprises one or more flexible plies having side edges wrapped around the beads and forming turn-ups terminating radially above the beads, and a pair of sidewalls. An apex filler is in contact with the outer periphery of each bead and extends radially outwardly therefrom between the carcass ply and the turn-up. The apex filler has a generally triangular cross-section comprising a base in contact with the bead and a pair of sides converging to form an apex radially above the base. The improvement of the present invention involves forming the apex filler by extrusion and immediately thereafter coiling the extrusion into a spiral of smaller inner diameter than the outer diameter of the peripheral surface of the annular bead. This coiling of the extrusion causes the base of the extrusion to be compressed and the peak of the extrusion to be stretched while maintaining the triangular cross-sectional shape of the extrusion. The inner diameter of the extrusion is between about 50% and about 75%, and preferably between about 50% and about 60% of the outer diameter of the annular bead. The coiled extrusion is then cooled while on a mandrel to maintain the coiled shape after which the extrusion is wrapped around the outer periphery of the annular bead to form the apex filler/annular bead subassembly.
The present invention further involves a method of producing an apex filler comprising extruding an elastomeric compound through a die to form an extrusion having the cross section of a triangle with a base and a pair of sides converging toward a peak. The method comprises forming the extruded elastomeric compound into a spiral configuration immediately following extrusion. The result is that the base of the extrusion is subject to compression and the peak of the extrusion is subject to tension while maintaining the triangular cross-sectional shape of the extrusion. The extrusion is formed into a spiral by wrapping it around a rotating mandrel. The diameter of the mandrel is between about 50% and about 75%, and more preferably between about 50% about 60% of the outer diameter of the annular bead. The extrusion is maintained in its triangular, extruded shape while being wrapped around into a spiral coiled shape on the rotating mandrel. The coiled shape is maintained by cooling or heating the extrusion while on the rotating mandrel. The extrusion is then transferred to the outer periphery of the bead to form the apex filler/annular bead subassembly.
The invention also encompasses an apparatus for use in relieving the stresses in a triangular elastomeric extrusion having a base and a pair of sides converging toward a peak. The extrusion is adapted to be used as an apex filler for making an apex filler/annular bead subassembly for a pneumatic tire. The apparatus comprises a mandrel having a diameter between about 50% and about 75% of the outer diameter of the annular bead, means to rotate the mandrel, means for changing the temperature of the mandrel to heat or to cool the extrusion wrapped therearound, and means for moving the extrusion in an axial direction along the mandrel as the mandrel rotates. The mandrel includes a helical former radially surrounding the cylindrical surface of the mandrel. The purpose of the helical former is to maintain the triangular cross-sectional shape of the extrusion as it is transformed from a linear extrudate into a spiral helix having a base in compression and a peak in tension. The helical former comprises an axially inverted, V-shaped opening corresponding to the cross sectional shape of the extrusion. The interior surface of the former comprises a low friction surface such as Teflon, or is coated with a low friction coating. As an alternative, the helical former may comprise a plurality of rollers along both sides of the helical pathway. The rollers are aligned in the plane transverse to the axis of the mandrel with the rollers in one row inclined toward the rollers in the other row to form the inverted V-shaped opening corresponding in shape and size to the cross-sectional shape of the extrusion.
The invention also includes a combination of a bead and an apex filler, made according to the process as herein described, for use in a pneumatic tire. The apex filler has a generally triangular cross section comprising a base and a pair of sides extending radially outward from the base and converging to form a peak. The base of the apex filler is in contact with the outer periphery of the annular bead, and the sides of the filler extend radially outward therefrom. The combined bead and filler are produced by forming an extrusion and then immediately thereafter coiling the extrusion into a spiral having an inner diameter between about 50% to about 75%, and preferably between about 50% to about 60% of the outer diameter of the annular bead. The coiling subjects the base of the extrusion to compression and the peak of the extrusion to stretching while maintaining the triangular cross-sectional shape of the extrusion. The extrusion is then wrapped around the outer periphery of the annular bead to form an apex filler/annular bead subassembly. Preferably, the coiled extrusion is cooled on the mandrel before being wrapped around the annular bead.